The study showed that young people who are exposed to sex on screen tend to lose their virginity at an earlier age and be more promiscuous
Children who watch films with a high sexual content tend to lose their virginity earlier and have more partners, a study has found.
Not only are they more promiscuous, they are also more likely to engage in risky sex by not using condoms.
The six-year study of more than 1,200 teens refers to sexual content in films but campaigners against online porn say it could equally apply to videos on the internet.
They point out that children can now see a lot more sexual imagery online than they ever did at the cinema - meaning that the effect will be magnified. Pornographic images and videos are freely available on the net, many on sites with no age verification procedures at all, putting children at risk.
The NSPCC warns that young male teens are now pressurising their girlfriends to copy what they see on porn films downloaded from the internet.
Other reviews have found that exposure to porn makes boys more likely to view girls as sex objects.
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is campaigning for an automatic block on online porn. Over-18s would be able to see adult material only if they specifically opted in.
The study on the effect on younger teenagers of sex in films was published in Psychological Science, the peer-reviewed journal of the Association for Psychological Science.
Researchers from Dartmouth College, an Ivy League university in the US, surveyed 684 top-grossing movies from 1998 to 2004, and then coded them for sexual content.
A film such as Eyes Wide Shut would be coded highly, while Lord Of The Rings: The Return Of The King would get a low coding. Although some of the films had 18-ratings, they could have been seen by younger children on DVDs at home.
The researchers then recruited 1,228 youngsters aged between 12 and 14, and each was asked which films on the list they had seen from a number of different collections of 50 that were randomly selected.
Six years later the same participants were surveyed to find out how old they were when they became sexually active and how risky their sexual behaviour might have been.
The results indicated that exposure to sexual content in movies at an early age is likely to influence adolescents' sexual behaviour.
Lead researcher Dr Ross O'Hara said: 'Much research has shown that adolescents' sexual attitudes and behaviours are influenced by media. But the role of movies has been somewhat neglected, despite other findings that movies are more influential than TV or music.'
Speaking about the results of his study, Dr O'Hara said: 'Adolescents who are exposed to more sexual content in movies start having sex at younger ages, have more sexual partners, and are less likely to use condoms with casual sexual partners.'
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The researchers said movies could have this effect on adolescents because of a personality trait they dub 'sensation-seeking'.
Between the ages of ten and 15, the tendency to seek more novel and intense stimulation of all kinds peaks. The wild hormonal surges of adolescence makes judicious thinking a bit more difficult.
Dr O'Hara said that greater exposure to sexual content in movies at a young age actually led to a higher peak in 'sensation-seeking' during adolescence.
He said: 'These movies appear to fundamentally influence their personality through changes in sensation-seeking, which has far-reaching implications for all of their risk-taking behaviours.'
And although Dr O'Hara said it was impossible to prove a direct causal effect between such films and sexual behaviour, he added: 'This study, and its confluence with other work, strongly suggests that parents need to restrict their children from seeing sexual content in movies at young ages.'
Pippa Smith, of pressure group Safermedia, said the findings were just as relevant to online images as they were to movies.